1. Field of the Invention
This invention relates to the continuous casting of metal ingots. Particularly, the invention relates to the casting of aluminum alloy ingots in an electromagnetic mold. More particularly, the invention relates to an apparatus and method for improving the surface characteristics of an aluminum-magnesium alloy ingot by inhibiting the formation of vertical folds, oxide releases and other surface defects thereon.
2. Description of the Prior Art
It is well known to continuously cast metal or metal alloy ingots. A typical continuous ingot caster comprises a containment or metal trough positioned over a casting mold. The casting mold includes a bottom plate or block positioned in close proximity to a plurality of mold sidewalls joined along adjacent edges. When these sidewalls are exposed to a coolant, the molten metal deposited therebetween solidifies from bottom to top and from side surfaces inwardly. As the metal solidifies, the bottom block is withdrawn from the sidewalls to continuously discharge the ingot out the bottom of the mold.
There are several disadvantages to casting ingots of an aluminum alloy in the above manner. As these ingots solidify, their side surfaces adhere to the mold sidewalls and tear away unevenly. Other surface defects such as laps and inverse segregation also result from ingot contact with the mold sidewalls. Before these ingots may be rolled into various sheet products, the rough side surfaces must be cut smooth or scalped. Scalping is costly, however, in terms of the machinery and manpower required, production time lost and potentially usable metal removed from each ingot.
It is further known to continuously cast metal ingots in a mold which includes an electromagnetic field. More particularly, an electromagnetic casting mold includes a bottom block extending to or between an electromagnetic inducting surface. The molten metal delivered to this mold assumes a shape defined by the inducting surface. Particularly, electromagnetic forces from the inducting surface repel molten metal inwardly until it sufficiently solidifies with exposure to a coolant. Ingots are cast, therefore, without contacting any physical surface other than the mold bottom block. These ingots typically have smooth side surfaces which do not require scalping. They also exhibit a more uniform chemical composition and crystalline structure.
In the electromagnetic continuous casting of some aluminum alloys, the oxides which should deposit as a thin, uniform film on the side ingot surfaces collect on the upper molten surface during casting in the mold. These metal oxides then release over the meniscus between the upper molten surface and side ingot surfaces and deposit as thick bands or thereon. Oxide releases diminish the appearance and value of the ingots on which they form. Hence, such releases are typically removed before rolling or resale by scalping.
For billets or ingots electromagnetically cast in a cylindrical shape, there are fewer problems with oxide releases because oxides tend to shift more smoothly over the nearer meniscus of said ingots and deposit more uniformly thereon. Nevertheless, there is disclosed a shell-like apparatus for inhibiting the formation of uneven oxide layers on such ingots in U.S Pat. No. 4,273,180. This apparatus extends perpendicularly from a mold cover and partially into the upper molten surface to minimize the surface defects caused by electromagnetic turbulences from the mold inducting surfaces. The apparatus further shapes the side surfaces of the ingot and seals the upper molten surface of same in a protective atmosphere of flux or slag.
It is further known to cover the solidification zone of an electromagnetic casting mold in order to maintain an atmosphere of nonreactive gas, such as nitrogen or argon, thereunder. Exemplary of said covers is that disclosed in Russian Patent No. 455,794.
In the electromagnetic continuous casting of substantially rectangularly-shaped ingots, oxide release problems are more prevalent. Although metal oxides should deposit as a thin, uniformly, outer film on said ingots, they tend to gather on the upper molten surface and release irregularly to form thick bands or patches thereon.
To promote a more uniform deposit of oxides on continuously cast aluminum alloy ingots, a floating skim dam or frame was disclosed in Japanese Patent No. 54-40210. A preferred embodiment of the skim frame taught therein resembles a cross section of the ingot cast and consists of two (2) U-shaped sections of insulating material, each section having outer lateral edges which contact with the ingot at an angle between 30.degree. and 60.degree. . Most particularly, the outer lateral edges of said skim dam intersect the upper molten surface at a 45.degree. angle between 0.4-6.0 inches (10-150 mm) inwardly from the side ingot surfaces.
For aluminum-magnesium alloy ingots, the oxide release problem is further complicated by the ease at which magnesium oxide forms. When the foregoing shell-like apparatus or converging skim frame is extended into an upper molten surface of said ingots during casting, overall surface characteristics are not substantially improved. Although these prior art devices often inhibit the formation of oxide releases, they also cause an undesirable amount of grooves or vertical folds to form. Such vertical folds seldom exceed 0.3 inch (7.6 mm) in depth. Because they interfere with efficient rolling of the ingots, however, they must also be removed by scalping.